Recently, for wireless communication systems, schemes are being investigated to eliminate dead zones, expand cell coverage, increase average reception strength, and improve throughput by arranging small-scale cells, such as microcells and picocells. Generally, mobile stations measure the strength of signals transmitted from base stations and implement control to connect to a cell having greater reception strength.
Consequently, when transmission strengths and cell sizes are respectively identical for all base stations, by control above, a mobile station selects an optimal cell for communication with respect to the uplink and downlink. An optimal cell is, for example, a cell for which the reception strength at the mobile station is greatest for the downlink and the reception strength at the base station is greatest for the uplink.
On the other hand, in communication systems where the cell sizes and transmission strengths differ among base stations, at a position between a large cell and a small cell, the cell for which the uplink reception strength is greatest and the cell for which the downlink reception strength is greatest may differ. This is consequent to the downlink reception strength measured at the mobile station being dependent on 2 factors, the transmission strength of each base station and downlink propagation loss, whereas the uplink reception strength measured at the base station is dependent on uplink propagation loss.
For example, at the mobile station, downlink reception strength is greater for larger cells, which have greater transmission strength; and at the base station, the reception strength of signals from mobile stations is greater for smaller cells since mobile stations are closer in distance and suffer less propagation loss. Under such circumstances, conventionally, when a cell for a mobile station is determined according to the reception strength of signals from base stations, the downlink reception quality improves, but conversely, the uplink reception quality diminishes.
To address this problem, technology has been disclosed in which the base station and the mobile station have a function of performing communication with different cells for the uplink and downlink (see, for example, Published Japanese-Translation of PCT Application, Publication No. 2007-514367; Japanese Laid-Open Patent Publication No. H6-224828; and MORIMOTO, Akihito; TANNO, Motohiro; KISHIYAMA, Yoshihisa; MIKI, Nobuhiko; TAOKA, Hidekazu; HIGUCHI, Kenichi; and SAWAHASHI, Mamoru, “Downlink/Uplink Radio Link Connection Methods in Heterogeneous Network for LTE-Advanced”, 2008 IEICE Conference, Sep. 16, 2008, p. 327). For example, for the downlink, communication is performed with a cell having greater reception strength at the mobile station and for the uplink, communication is performed with a cell having less propagation loss and greater reception strength at the base station.
However, with the conventional technologies above, by making uplink and downlink connections with different cells, control information overhead increases, triggering deterioration of communication efficiency. For example, in various types of wireless communication schemes such as Wideband Code Division Multiple Access (W-CDMA) and Long Term Evolution (LTE), a large part of the control information exchanged between the mobile station and the base station is for wireless interval control.
This control information includes, for example, feedback information for acknowledgment of transmitted data and notification of wireless resource allocation. In the uplink and the downlink, if communication is limited to one direction, since the control information is forwarded by a physical communication line between base stations, the scheduling process at each of the base stations becomes complicated. For example, when control information is forwarded among the base stations, the scheduling process at each base station is difficult to perform independently for each base station. Further, delays in the scheduling process occur consequent to the forwarding of control information.